The present invention relates to a nondestructive inspection apparatus capable of inspecting an internal defect existing in a measuring object such as a concrete structure, and more particularly it relates to a nondestructive inspection apparatus which enables the diagnosis of internal defects with high reliability by correcting the reflection energy level of an acoustic elastic wave in the form of a vibration wave with high accuracy.
In the past, hammering tests from an external surface using a hammer have been widely practiced in order to detect internal defects existing in a concrete structure for instance for the reason of convenience and facilitation.
However, when such a well-known hammering test method is used, the accuracy of tests depends on the operator""s ability and the level of his or her skill, and it is extremely difficult to carry out hammering by a constant force at all times. In addition, there has been a problem in that the criterion for determination depends greatly on the operator""s experience and intuition, so the result of diagnosis becomes vague, making it impossible to achieve sufficient reliability.
Thus, in order to obviate the above problem, there have been proposed a variety of kinds of improved nondestructive inspection apparatuses.
FIG. 11 is a block diagram illustrating a known nondestructive inspection apparatus described for instance in Japanese Patent Application Laid-Open No. 8-21824 to which a method of inspecting defective fillings is applied.
In FIG. 11, the nondestructive inspection apparatus includes a wave transmitting probe 111 which constitutes an acoustic wave transmitting section, a wave receiving probe 112 which constitutes an acoustic wave receiving section, a survey unit 113 having an acoustic wave transmitter, and an FFT analyzer 114 which executes calculation processing by using fast Fourier transform.
Also, a nondestructive inspection object (i.e., an object to be subjected to nondestructive inspection) is provided with a plate member 120 against which the wave transmitting probe 111 and the wave receiving probe 112 are adapted to be placed into abutment, and a filler 130 such as concrete, etc., arranged in the plate member 120. The filler 130 includes a filling defective portion 131 such as a crack, a void, etc., as shown in FIG, 11.
Now, reference will be made to a method of inspecting defective fillings carried out according to the known nondestructive inspection apparatus illustrated in FIG. 11.
First of all, the wave transmitting probe 111 for acoustic wave transmission and the wave receiving probe 112 for acoustic wave reception are placed into abutment against a surface of the plate member 120, as shown in FIG. 11, and an acoustic wave having a wide-band frequency component is repeatedly transmitted from the survey unit 113 through the wave transmitting probe 111.
As a result, the acoustic wave transmitted from the wave transmitting probe 111 is repeatedly sent from the surface of the plate member 120 toward the filler 130.
The wave receiving probe 112 receives a reflected wave of the acoustic wave transmitted from the wave transmitting probe 111 and converts it into a corresponding electric signal, which is then input to the FFT analyzer 114 through the survey unit 113.
The FFT analyzer 114 carries out Fourier analysis of the reception signal and outputs a frequency spectrum level thus obtained to a CRT display (not shown), etc.
Accordingly, an operator can measure the frequency spectrum level output by the FFT analyzer 114 so as to determine the presence or absence of the filling defective portion 131.
With the known nondestructive inspection apparatus as constructed above, if each of the probes 111, 112 is not in contact with the surface of the plate member 120, which acts as a measuring surface, in a satisfactory manner, there would be generated attenuation of the acoustic elastic wave between the wave transmitting probe 111 (vibrating section) or the wave receiving probe 112 (receiving section) and the contact surface (measuring surface) of the plate member 120, thus making it difficult to accurately measure the reflection energy level.
Particularly, the surface condition of the concrete structure is variously changed depending upon the environment where it is placed, so there will likely arise a situation that each of the probes 111, 112 is not able to contact the measuring surface to any satisfactory extent. Such a situation may be considered to include the cases wherein the measuring surface is rugged, or is deteriorated by weathering, or is attached by dust or the like for example.
Moreover, in cases where the respective probes 111, 112 are manually pushed against the measuring surface, the ruggedness of the measuring surface and the condition of attachment of foreign matters greatly influence the contact forces, thereby further reducing the accuracy of measurements by the use of the level of the reflected wave.
In addition, with the known nondestructive inspection apparatus, in cases where a measuring object is changed, or the condition of the contact surface varies with the lapse of time, it would be impossible to make comparison of reflected waves, and hence it has been difficult to evaluate different objects through comparison or by following or tracing changes thereof over time.
Furthermore, since the reflection energy level of the reflected wave cannot be compared by using a constant criterion, there has been a problem that it is impossible to determine a correlation between the distance of the filling defective portion 131 of the filler 130 to the reflection surface and the reflection energy level.
The present invention is intended to obviate the problems as referred to above, and has for its object to provide a nondestructive inspection apparatus which can infer the condition of the contact of a vibrating section and a receiving section with a measuring surface thereby to correct a criterion, and enable the comparison of the absolute reflection energy level even if there is poor contact of the vibrating section and the receiving section with the measuring surface, thus substantially improving the accuracy of measurement of the reflection energy level irrespective of the surface condition of a measuring object and at the same time making it possible to calculate the distance of an internal defect in the measuring object from the measuring surface thereof by using a correlation between the distance from the surface to a filling defective portion (internal defect) in the measuring object and the reflection energy level.
The present invention resides in a nondestructive inspection apparatus for diagnosing an internal defect of a measuring object by injecting an acoustic elastic wave into the measuring object, the apparatus comprising: a vibrating section which is adapted to be placed in pressure contact with a surface of the measuring object for generating the acoustic elastic wave; a receiving section which is adapted to be placed in pressure contact with a surface of the measuring object for receiving a reflected wave of the acoustic elastic wave; a pushing mechanism for pushing the vibrating section and the receiving section against the surface of the measuring object; a pushing force measurement section for detecting pushing forces of the vibrating section and the receiving section against the surface of the measuring object during vibration thereof; a vibration control section for driving the vibrating section thereby to generate the acoustic elastic wave; and a reception signal processing section for determining the internal defect based on the reception signal from the receiving section; wherein the reception signal processing section comprises: a reflection energy calculation section for calculating a reflection energy level due to elasticity vibration of the measuring object based on the reception signal; a reflection energy correction section for normalizing the reflection energy level by the pushing force to calculate a correction value; and an internal defect determination section for detecting the internal defect based on the correction value.
Moreover, the vibrating section according to the nondestructive inspection apparatus of the present invention includes a magnetostrictive vibrator for generating the acoustic elastic wave through a magnetostriction phenomenon.
In addition, the acoustic elastic wave according to the nondestructive inspection apparatus of the present invention comprises a chirp wave with its frequency continuously changing with time; the reception signal processing section includes an envelope detecting section for determining an envelope of elasticity vibration caused by the reflection of the chirp wave, the envelope detecting section being operable to calculate, based on the envelope, a resonance frequency according to a natural oscillation characteristic of the measuring object; and the internal defect determination section detects the internal defect based on the resonance frequency and a response waveform of the supply frequency.
Further, the internal defect determination section according to the nondestructive inspection apparatus of the present invention calculates a distance to the internal defect based on a correlation between a distance to the internal defect and the correction value which is prepared in advance.
Furthermore, the correlation between the distance to the internal defect and the correction value according to the nondestructive inspection apparatus of the present invention is stored in advance in the internal defect determination section as map data of actual measurement values corresponding to the measuring object.
Still further, the reflection energy correction section according to the nondestructive inspection apparatus of the present invention calculates an additional correction value by dividing the correction value by an abnormal range area of the internal defect; and the internal defect determination section calculates the distance to the internal defect based on a correlation between the distance to the internal defect and the additional correction value which is prepared in advance.
Moreover, the correlation between the distance to the internal defect and the additional correction value according to the nondestructive inspection apparatus of the present invention is stored in advance in the internal defect determination section as map data of actual measurement values corresponding to the measuring object.
In addition, the measuring object according to the nondestructive inspection apparatus of the present invention comprises a concrete structure.